The present invention relates to a holographic display with an illumination device, an enlarging unit and a light modulator. The holographic display serves to present two-dimensional and/or three-dimensional image information.
There are two major problems one faces when realising holographic displays with large screen size:                If a large-area light modulator, e.g. having a diagonal measurement of 24″, is used to encode the hologram, then this large area of the light modulator must be illuminated uniformly with sufficiently coherent light.        In contrast, if a small light modulator is combined with a projection arrangement, then the device will be deeper than 1 m if it has the same screen size of 24″ and if conventional optical means such as lenses and mirrors are used.        
It should be possible to solve the first problem with a large illumination device that is as flat as possible. The second problem can only be solved if other means than conventional optical means are used to enlarge the illumination device and/or the light modulator.
A holographic projection display is disclosed for example in document WO 2006/119760 A2. In that document, a light modulator with a small surface and high resolution, on which a hologram is encoded, is imaged in an enlarged manner with the help of an arrangement of lenses and mirrors onto a lens or concave mirror which serves as a screen and reconstructed in a space which stretches between the screen and a viewing window which is situated in the Fourier plane of the screen. Thanks to the enlarged imaging of the hologram onto the screen, that arrangement has the advantage that the reconstruction space is enlarged too, so that much larger objects can be reconstructed than in conventional holographic arrangements. However, this goes along with the disadvantage that the optical arrangement is rather voluminous and particularly long in the axial direction, so that it can hardly be used as a holographic desktop display because of its large depth.
In the projection display which is disclosed in document US 2007/252956 A, a small light modulator is illuminated by a relatively small illumination device and projected in an enlarged manner onto a screen with the help of an extra-axially disposed holographic mirror element. It is an advantage of that arrangement that the axial dimension of the entire system is shortened because of the oblique optical path. However, the arrangement is still too voluminous to be used as a holographic desktop display.
Document WO 2002/082168 A discloses a flat projection display which combines a one-dimensional and a two-dimensional grating for light deflection. The virtual image of a video projector is guided through a rod-shaped grating body in one direction, and it is then guided through a plate-shaped grating body in a second direction which is perpendicular to the first one. In one embodiment, the gratings are made of glass strips which are joined in layers at an angle of 45° to the surface of the display, each of which deflecting the light at right angles to the direction of incidence. However, the image of the light modulator is thereby rather multiplied than enlarged, and an observer who looks at the surface of the plate-shaped grating body in the normal direction sees a two-dimensional arrangement of one and the same modulator image. A holographic projection display where the encoding surface of the light modulator is actually enlarged cannot be realised with such an arrangement though.
In document WO 2002/31405 A, a collimated pencil of light rays with rectangular cross-section, which is for example emitted by a light modulator, is broadened in two perpendicular directions in that it falls at a small angle on a one-dimensional surface that is not reflecting like a mirror and on a two-dimensional surface that is not reflecting like a mirror one after another. The two-dimensional broadening is achieved through the flat, “grazing” incidence, and the surfaces have such texture that they reflect the light beams into the desired direction, that is perpendicular to the direction of incidence in this case. This is realised with the help of two-dimensional diffraction gratings or holographic surface gratings. In that arrangement, the cross-sectional area of the incident light wave field is truly enlarged, but there is no mention of defined amplitude and phase modulation of the pencils of light rays when they are reflected by the diffraction gratings, which would, however, be essential in the context of holographic reconstruction of three-dimensional scenes.